Anyone had trouble with this, it's happened couple of times but only for very short periods, when iv bin going down the road in fifth, get to about 50mph and it seems to start revving higher but. Nt actually any quicker, then I seem to dip the clutch. And it stops ?

Probably a bit of crud in the bore of the master or the slave making the piston stick. As the clutch wears the pistons travel a little further. Try whipping them off and cleaning the bores carefully with some very fine wet and dry